I always use png now
no muddy jpg, no 'unreliable' gif
quite small too

The advantage of the GIF format is that it is paletted, which means that if
an image has fewer colors, you can use a smaller, selective palette. This
reduces the size of the file without compression. The smaller the number of
colors in the palette, the smaller the image. PNG is not universally
supported, and is not particularly small in size. It's greatest selling
point is its ability to store variable alpha (transparency). GIF has alpha,
but uses a single color for transparency, and the transparency is either off
or on for any pixel, rather than variable. So, assuming that your graphs
have only a few colors in them, GIF might be your best bet.